Crop Photosynthesis. Spatial and Temporal Determinants

455 Pages·1992·13.758 MB·English

by  N.R. Baker and H.C. Thomas (Eds.)

Topics in Photosynthesis, Volume 12
series editor
J. Barber
Department of Pure and Applied Biology,
Imperial College, London SW7 2BB, U.K.
ELSEVIER
AMSTERDAM · LONDON · NEW YORK · TOKYO
Crop Photosynthesis: spatial and
temporal determinants
edited by
N.R. Baker
Department of Biology,
University of Essex;
Colchester CÖ4 3SQ, Essex, U.K.
and
H. Thomas
Institute of Grassland and Environmental Research,
Pias Gogerddan,
Aberystwyth, Dyfed SY23 3EB, U.K.
1992
ELSEVIER
AMSTERDAM · LONDON · NEW YORK · TOKYO
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v
Obituary
NORMAN E. GOOD
(1917-1992)
Norman Everett Good was born May 20, 1917 in Brantford, Ontario where he spent most of his
first thirty years tending livestock and fruit trees on the family farm. After completing his
undergraduate degree at the University of Toronto in 1948, he attended graduate school at the
California Institute of Technology and, in 1951, received his Ph.D. for research with Herschell Mit­
chell on amino acid metabolism in Neurospora. War years resulted in several interruptions of his
education during which he returned to work on the family farm. He sometimes wondered (Good,
1986) whether these interruptions might not have been responsible for making academic life seem
to him more like a game than like working. If Professor Good's professional career was a game
then he was surely a world-class player. Because of his personal modesty and the rapid incorporation
of his most significant research contributions into the scientific 'public domain', very few, including
ourselves, are aware of the full scope and significance of this man's career-long contributions. We
cannot hope to fill that gap here but rather, we offer our modest personal perspectives and tributes
to a man who was our mentor and whom we will greatly miss.
It was not until his postdoctoral days that Norman Good became involved with the photosynthesis
research that was to become the major focus of his career. With Allen Brown at the University of
Minnesota, he used 180 as a substrate for respiration and the then new technique of mass spec-
2
trometry to differentiate respiration from concurrent photosynthesis, thereby establishing the elec­
tron stoichiometry of photosynthetic oxygen exchange (Brown and Good, 1955). However, by his
own account (Good, 1986), the most influential period shaping his professional life began in 1952
when he arrived in Cambridge to work with Robin Hill. Together, Hill and Good developed the use
of flavins and viologens as electron acceptors for monitoring electron transfer in isolated thylakoids
(Good and Hill, 1955). In connection with this work, he devised simple yet highly accurate pro­
cedures to measure photosynthetic electron transport in isolated chloroplast membranes. This work
permitted the manipulation of the electron transfer reactions and laid the groundwork for an enor­
mous amount of research in subsequent years throughout the world. After two years with Hill, Dr.
Good joined the Canadian Department of Agriculture where, in addition to continuing his
photosynthesis research, he began a study of indolacetic acid metabolism in plants and made the
first discovery of a conjugated auxin, indoleacetylaspartic acid (Andrea and Good, 1955). In 1958,
Dr. Good left Canada, though he always remained a Canadian national, and began a distinguished
career as a faculty member at Michigan State University. In 1978, he received that university's
Distinguished Faculty Award, and in 1988, the American Society of Plant Physiologists' Charles
F. Kettering Award for Excellence in Photosynthesis.
Not long after arriving in East Lansing, work in Good's laboratory suggested that two segments
of the photosynthetic electron transfer chain were coupled to ATP formation (Winget et al., 1965).
During the next ten years while this notion was still controversial, he and his students were almost
VI
solely responsible for the development of techniques that allowed functional isolation of various
segments of the chain (e.g. Saha et al., 1971). The importance of this work to subsequent advances
toward understanding the mechanism of electron transfer and energy coupling is inestimable, and
in 1973, it provided the incontrovertible evidence for two coupling sites (Izawa et al., 1973). As the
photosynthesis research community became aware of the existence of the coupling factor complex
of the thylakoid membrane, Good's lab led the way in the structured - function relationship of a
class of compounds which inhibited its activity (Winget et al., 1969). Over the years, this laboratory
provided some of the most important evidence in support of the major facet of chemiosmotic
mechanism of energy coupling, that is, the essential role of the transmembrane proton activity dif­
ference in energizing ATP formation (e.g. Izawa et al., 1974; Ort et al., 1976). In spite of this, Dr.
Good chose to play the role of devil's advocate and continually insisted that the detailed
mechanism of coupling be experimentally addressed (e.g. Good, 1988).
Good's group is also credited with pioneering work in thylakoid membrane organization. The
widely cited publication by Izawa and Good (1966) marked the discovery of ionic strength-
dependent, reversible unstacking of thylakoid grana, thereby stimulating yet another new research
area which has seen a great deal of activity and excitement over the past two decades.
Prior to a seminar presented at Purdue University in 1974, David Krogmann introduced Norman
Good as the single individual who had touched the research lives of more biochemists and
physiologists than anyone in history through the development of the 'Good buffers'. Indeed, these
dipolar ionic buffers are so extensively used, particularly in biomedical research, they outrank every
other biochemical reagent in value of sales. Although he often dismissed this extraordinary ac­
complishment (Good et al., 1966) as "a bit of undergraduate chemistry", there was nothing trivial
about the design, the synthesis or the biological compatibility testing of these revolutionary
hydrogen ion buffers. That only a small fraction of these novel compounds made the grade was evi­
dent to anyone who spent time in the Good laboratory where drawers were filled with baby food
jars of white powders, each labeled with its own arcane acronym (Dr. Good claimed that some of
these were abandoned simply because they could not be pronounced). Although his droll wish for
a student named Better or Best was never to happen, he did design a second generation of these
compounds (Ferguson et al., 1980) when it became known that precursors in certain of the original
buffers were possibly carcinogenic.
Norman Good recognized that the importance of buffering in biological research extended much
beyond hydrogen ions. Along these lines, he proposed using IAA-amino acid conjugates to buffer,
and thus stabilize, the auxin concentration in tissue culture media. A wide range of IAA-amino acids
were synthesized, several of which have been used to advantage in various culture systems (e.g.
Hangarter et al., 1980). This work also lent support to the hypothesized role of phytohormone con­
jugates in maintaining hormonal homeostasis in plants. In the last years before his retirement, Good
pursued his ideas on the design of compounds that could be used to buffer, and thereby control,
concentrations of specific metals.
For many, Norman Good was a guru of plant biology. His lab was a frequent gathering place
for students and faculty seeking his exceptional insights into their research. There seemed to be no
limit to the time or patience that he would devote in this way. His insight came not from a large
number of accumulated facts but rather from an understanding and biologically relevant apprecia­
tion of fundamental principles (this was also the basis for his fame, some students might claim in­
famy, on preliminary examination committees). For many of us, he was the consummate editor;
there is little doubt he could have improved this effort considerably.
For all his academic success, Dr. Good never lost touch with his agrarian background. His
students and colleagues will not forget the apples and grapes that he tended from spring to fall.
Photosynthesis researchers in the midwestern U.S. will long associate our annual meeting with the
bushels of apples he supplied for our enjoyment. The death of Norman Good leaves us all feeling
an enormous loss and his students with the sense of the passing of a scientific generation.
Vll
REFERENCES
Andrea, W.A. and Good, N.E. (1955) Plant Physiol. 30, 380-382.
Brown, A.H. and Good, N. (1955) Arch. Biochem. Biophys. 57, 340-354.
Ferguson, W.J., Braunschweiger, K.I., Braunschweiger, W.R., Smith, J.R., Justin McCormick, J., Wasmann, C.C., Jarvis,
N.P., Bell, D.H. and Good, N.E. (1980) Anal. Biochem. 104, 300-310.
Good, N.E. (1986) Ann. Rev. Plant Physiol. 37, 1-22.
Good, N. and Hill, R. (1955) Arch. Biochem. Biophys. 57, 355-366.
Good, N.E., Winget, G.D., Winter, W., Connolly, T.N., Izawa, S. and Singh, R.M.M. (1966) Biochemistry 5, 467-477.
Hangarter, R.P., Peterson, M.D. and Good, N.E. (1980) Plant Physiol. 65, 761-767.
Izawa, S. and Good, N.E. (1966) Plant Physiol. 41, 544-552.
Izawa, S., Gould, J.M., Ort, D.R., Felker, P. and Good, N.E. (1973) Biochim. Biophys. Acta 305, 119-128.
Izawa, S., Ort, D.R., Gould, J.M. and Good, N.E. (1974) Proceedings of the Third International Congress on Photosynthesis
(Avron, M., ed.), pp. 449-461 Elsevier, New York.
Ort, D.R., Dilley, R.A. and Good, N.E. (1976) Biochim. Biophys. Acta 449, 108-124.
Saha, S., Ouitrakul, R., Izawa, S. and Good, N.E. (1971) J. Biol. Chem. 246, 3204-3209.
Winget, G., Izawa, S. and Good, N.E. (1965) Biochem. Biophys. Res. Commun. 21, 438-443.
Winget, G., Izawa, S. and Good, N.E. (1969) Biochemistry 8, 2067-2074.
R.P. Hangarter
D.R. Ort
IX
Foreword
Plant growth represents the excess of photosynthesis over respiration and therefore an increase in
net photosynthesis is tantamount to an increase in plant productivity. Since the plant is a closed
system, growth and net photosynthesis are inevitably the same and it is not self-evident which of
the companion processes determines the ultimate size of the organism. We must look more deeply
at the relationship of these phenomena if we are to analyze the effects of environmental stresses on
productivity.
Photosynthesis, nitrogen reduction, water and mineral uptake provide the supply of metabolites
that serves as the raw material for growth. Nevertheless the actual magnitude of a plant or animaPs
growth and its form and function depends on signals contributed by the DNA of the particular
species. The growth of any organism can be modified to a certain extent by the supply of car­
bohydrates from photosynthesis or other sources, but that is not the usual or most important factor
in determining ultimate size. Starved elephants are smaller than well-fed elephants but well-fed
aphids are much smaller than elephants regardless of nutrition. Similarly sunflowers are much larger
than mosses but it does not follow that sunflower photosynthesis is inherently more efficient. In
both cases the major limitations reside in the nature of the restriction on growth that, in turn, de­
pend on information from the genes.
This observation is the consequence of a fundamental engineering principle. When a variety of
processes in any complex machine draw on a common supply, the processes must be use-limited,
not supply-limited. For example, one cannot hope to regulate the electrical functions of a motor-car
by regulating the input from the storage battery. If we tried to do so, turning off the lights would
also disconnect the ignition, the horn, the windshield-wiper and the starter. Similarly, biological and
morphological differentiation in plants would not be possible if the overall supply of carbohydrates
from photosynthesis constituted a major control of metabolic processes. This biological analogy to
the electrical system of a car is not trivial. It embodies a concept that must be kept in mind always.
However, before we invoke the concepts of supply and use in discussing photosynthesis and growth,
we must define the terms rigorously: supply depends on photosynthesis but not until the photosyn-
thate has been transported, processed through many intermediates and combined with nitrogen and
minerals. Only then are the raw materials available for the imprint of the genes that determine form
and function.
Incidentally, this inclusion of translocation as part of the supply equation and the incompatibility
of supply limitations with differentiation show the futility of trying to interpret morphological
modifications and the harvest index in terms of translocation. Translocation differences must be
primarily a result, not a cause, of differentiation.
How do plants adjust the quality and quantity of growth without exceeding the supply of car­
bohydrates and Starving' under stress? This is an area of great importance because photosynthetic
production of carbohydrates is variable and uncertain. Photosynthesis in the field is completely at
the mercy of the availability of water, light, carbon dioxide, reduced nitrogen and minerals. It is
rare indeed that these are all optimal throughout the life of the plant. Somehow the plant must
X
establish and maintain use priorities that persist over a wide range of conditions and a wide range
of growth rates. DNA-encoded signals from the genes set a pattern for the priorities within which
development must take place. Within these priorities, ever-changing environmentally regulated
development must modify growth and differentiation. Reversible or irreversible modifications of
metabolic activities must provide the fine-tuning of growth processes that we define as adaptation
to stress.
A primary task is to describe the responses of plants to various stresses. Then we must undertake
the much more difficult task of explaining how these stresses modify growth and, incidentally,
regulate photosynthesis. A word of caution may be needed here. Stress-related decreases in produc­
tivity can be subject to misinterpretation if control plants and experimental plants are grown under
standard unstressed conditions and then abruptly subjected to experimental treatments. Since field
adaptation to stress is in large part a developmental process, plants require time to adapt their
metabolism to new conditions. Sometimes the adaptations may require a quite extensive turnover
of plant components. Thus, although well-watered plants suddenly subjected to water deprivation
tend to cope with the new adversity by closing their stomates, thus avoiding disastrous dehydration,
this response may not be typical in nature. Indeed such experiments bear little relation to field condi­
tions; short-term exposures to stress preclude the long-term adaptations that play an important role
in adjustments to real-life deficiencies. Plants actually grown under water stress may grow less, use
less photosynthate and as a consequence build up the level of internal C0 . If they close their
2
stomates they may do so because of less growth rather than the converse.
On the other hand, long-term experiments are subject to another kind of misinterpretation even
if the control plants are properly chosen and the stressed plants are given ample time to adjust. If
the stressed plants are smaller, as they almost certainly will be, one must assume that, by definition,
there has been less photosynthesis. In view of the discussion above, one is tempted to conclude that
stress-induced inhibition of growth has caused a build-up of carbohydrates and a feed-back inhibi­
tion of photosynthesis. Such inhibitions can sometimes occur but it is not always proper to invoke
them. Growth inhibition, or more properly growth redirection, can occur if it is the production of
new photosynthetic machinery that is curtailed. Thus, stressed and unstressed plants with identical
rates of photosynthesis (on a leaf area basis) will be quite different in size if the stressed plants invest
less of their photosynthate in making photosynthetic machinery and more of their photosynthate
in storage material, roots, or vascular tissue. The exponential term in the growth equation, the rate
of compound interest, and therefore the overall long-term growth rate, will be quite different even
though the rate of C0 fixation may be identical.
2
Those of us who have devoted our lives to the study of the mechanism of photosynthesis naturally
are intrigued by prospects of improving photosynthesis by introducing genes using the ever-more-
plausible techniques of molecular biology. However, for the reasons discussed above, useful im­
provements in the biochemistry of photosynthesis may not be feasible since growth limitation, not
photosynthesis limitation, underlies so many of the problems of photosynthesis. In the short term
perhaps the greatest commercial gains can be made from directed mutations of crops that produce
or eliminate specific substances. If these substances can be synthesized at minor metabolic costs,
their production is immune to the engineering considerations raised above. Substances that make
apples redder or peaches sweeter, substances of great medicinal or commercial value, and substances
conferring disease resistance may be of this nature. A better harvest index may also be among such
directed genetic modifications if the genes responsible for determining morphological differences
can be resolved. Also, it may be possible to use the techniques of genetic engineering to modify
maturity dates and increase the geographical ranges over which many crops can be grown. At pre­
sent the possibility of improving photosynthesis in a search for greater productivity is not necessarily
a desirable or even feasible option. In this volume consideration is given to many of the abiotic and
biological factors, beyond the physical, chemical and physiological mechanisms of the process of
photosynthesis, that are involved in determining photosynthetic performance. Consequently the
XI
book should provide a useful source of information in the quest to resolve the complex relationships
between photosynthesis, growth and crop productivity, an understanding of which is essential to
evaluate the potential gains, if any, from genetic manipulation of the photosynthetic and associated
processes.
Norman E. Good
Xlll
Preface
In 1850 there was an average of approximately 11 hectares of agriculturally productive land for each
member of the world's population. One hundred years later this figure had been reduced by two
thirds. At the end of this century it is predicted that there will be little more than 1.5 hectares per
person. For the future of the human race it is essential to ensure that every photon falling upon
this shrinking area will be used as effectively as possible. Technological developments associated
with improved land use, plant nutrition and pest control have increased agricultural efficiency, but
often at great financial cost and to the detriment of the environment. The long-term consequences
of environmental mismanagement and pollution are becoming increasingly apparent and greater
political pressures are being applied constantly to limit these problems. Improvements in crop pro­
ductivity resulting from the development of crop varieties with increased yield characteristics will
make an important contribution to limiting further damage to the global environment. Since
photosynthetic performance is a fundamental determinant of yield in the vast majority of crops,
an understanding of the factors limiting photosynthetic productivity has a crucial role to play in
crop improvement programmes.
As a result of the rapid and exciting developments in molecular biology and genetics over the past
decade, there has been an increasing tendency to collapse the vast variety of sizes, shapes, functions
and responses of organisms into a conceptual model occupying a single dimension: the sequence of
nucleotide bases along the DNA of genomes. Theoretically, since the genetic code defines not only
an organism's spatial and temporal organisation, but also the nature of its responses to environmen­
tal stimuli, it can be argued that reading of this vital code should enable everything significant about
an organism to be determined. At present we are far removed from achieving this remarkable goal.
For the foreseeable future resolution of many of the complexities of living systems will require not
only molecular biological and genetical analyses, but also the rigourous application of the more
classical disciplines of biochemistry, physiology and ecology. This is particularly true for predictions
of the responses of organisms to environmental change, where changes in the expression of many
hundreds of genes, if not more, may be involved.
Photosynthesis, unlike the majority of physiological processes in plants, has been the subject of
extensive studies at the molecular level for many years. This reductionist approach has resulted in
the development of an impressive and detailed understanding of the mechanisms of light capture,
energy transduction and carbohydrate biosynthesis, processes that are clearly central to the success
of the plant and the productivity of crops. However, no rational person would seriously wish to
place programmes for improving the photosynthetic performance of crop plants solely in the hands
of molecular biologists, biochemists and biophysicists. A considerable proportion of the life of a
plant is determined by factors operating beyond the physiological limits defined by the Z scheme
and the photosynthetic carbon reduction cycle!
The aim of this volume is to examine in the widest context the factors determining the photosyn­
thetic performance of crops. The emphasis throughout the book is on the setting for photosynthesis
rather than the fundamental process itself. Beginning at the highest levels of organisation - com-